1. Field of the Invention
The present invention relates to a single fibre bidirectional optical transmission system and a single fibre bidirectional optical amplifier, and more in particular, it relates to a single fibre bidirectional optical amplifier for performing the collective amplification of a bidirectional optical signal by an optical amplifier in a single fibre bidirectional optical transmission to perform a bidirectional transmission by a single optical fibre.
2. Description of the Related Art
In a single fibre bidirectional optical transmission technology for performing a bidirectional transmission by one piece of an optical fibre, comparing to a two fibre bidirectional transmission in which two pieces of fibres perform single directional transmission, respectively, the number of optical fibres to be used is reduced one half. Hence, when the optical fibres are to be newly laid down, the lying fibres is reduced one half, and when dark fibres are used, the fibre charges are reduced one half. Consequently, it is possible to construct an economical system.
In optical communications, in order to realize a long distance transmission, an optical amplifier is used to amplify an optical signal attenuated due to a transmission path loss as it is as a light beam without performing an optic-electric (O/E) conversion and an electric-optic (E/O) conversion. The optical amplifier is characterized in that it does not depend on a bit rate and a signal format and is capable of collectively amplifying a wavelength multiplexing signal and, therefore, it is possible to realize a flexible and low cost network.
In general, as for the optical amplifier, an erbium doped fibre amplifier for entering an excited light together with a signal beam into the erbium doped fibre and amplifying the signal beam is used. This optical amplifier is manufactured in such a way as to amplify the light beam transmitting single direction, and its configuration is not so complicated. For this reason, the optical amplifier cannot be simply inserted into a single fibre bidirectional transmission path in which the optical signal is transmitted bidirectional in the single optical fibre.
Hence, though the configuration is complicated, various techniques have been proposed as the single fibre bidirectional transmission optical amplifier. For example, as shown in FIG. 17, there is a method available, in which signals 252, 262, 272 and 282 advancing up and down into different directions, respectively in single fibre bidirectional transmission paths 84 and 89 are separated by optical circulators 83, 85, 88 and 90, and are amplified separately up and down, respectively, by using conventional optical amplifiers 86 and 87, and after that, are multiplexed again by the circulators 83, 85, 88 and 90.
At this time, in the optical amplifiers 86 and 87, there is required an optical multiplexer for multiplexing the excited light and the signal beam. Further, in order to separate the signals 252, 262, 272 and 282 up and down in the single fibre bidirectional transmission paths 84 and 89, there is required an optical demultiplexer. There has been proposed a technique for commonly using the optical multiplexer and the optical demultiplexer by the optical circulator having four ports (for example, refer to patent document 1).
Further, there has been also proposed a configuration (for example, refer to patent document 2), in which the common use of the optical multiplexer and the optical demultiplexer is attempted by using a reflector when the excited light and the signal beam in the optical amplifier are multiplexed, or the configuration similar to this (for example, refer to patent document 3).
On the other hand, as for the method for amplifying the up and down optical signals without separating them, there is also available a technique, in which the erbium doped fibre is connected to a bidirectional transmission path so as to add the excited light bidirectional up and down, thereby amplifying the up and down optical signals in bidirection (for example, refer to patent document 4).
[Patent Document 1]
Japanese Patent Laid-Open No. 6-342950 (Pages 4 to 6, FIG. 1)
[Patent Document 2]
Japanese Patent Laid-Open No. 11-274625 (Pages 7 and 8, FIG. 1)
[Patent Document 3]
Japanese Patent Laid-Open No. 2002-118313 (Pages 5 and 6, FIG. 1)
[Patent Document 4]
Japanese Patent Laid-Open No. 3-92827 (Page 161, lower right column, Page 162, upper left column, FIG. 6)
When an optical communication system aims specially at a metropolitan area, it places the first priority on the cost, and it is desirable to use low cost products. According to the technique in which the up and down optical signals are separated and separately amplified respectively similarly as described in the patent documents 1, 2 and 3, two sets of the optical amplifiers are required up and down among the conventional single fibre bidirectional optical amplifiers. This makes the cost expensive and the device size large, and causes a problem in that the consumption power becomes large and the like.
Further, according to this technique, by combining the separation of the up and down signals of the bidirectional transmission and the multiplexing of the signal beam with the excited light of the optical amplifier, the optical multiplexing and demultiplexing elements are commonly used. Therefore, it is necessary to newly develop an element for exclusive use of the single fibre bidirectional optical amplifier, and this causes a problem in that the cost becomes expensive.
On the other hand, as for the technique for amplifying the optical signal without separating up and down optical signal, though there is a technique disclosed in the patent document 4, it is indispensable for the optical amplifier to insert an optical isolator so that an oscillation does not build up in the interior of the optical amplifier by a reflection occurred at a connecting point of the erbium doped fibre and the transmission path and in the transmission path. Consequently, there arises a problem in that such a configuration is not possible to realize.
Further, according to the conventional single fibre bidirectional optical amplifier, a vertically symmetrical transmission path is conceived so that the up and down optical signal is equally amplified. Thus, the optical amplifier has to be installed in the center of the single fibre bidirectional transmission path, and consequently, there is a problem in that an installing condition is limited.
Further, a transmission distance goes beyond 60 km, and in the case where the transmission rate is not less than 10 Gbps per one wavelength, not only an optical amplification, but also a dispersion compensation is required. However, there is a problem in that no consideration has been given to this so far.
Hence, the object of the present invention is to solve the above-described problems and to provide a single fibre bidirectional optical transmission system and a single fibre bidirectional optical amplifier which can attempt the extension of a single fibre bidirectional long distance at a moderate price.